The present invention relates generally to nuclear magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) and, more particularly, to prescribing pulse sequences for an MRI or MRS system that precisely target a selected structure to be examined.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A NMR signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image. The RF excitation pulse that produces this B1 excitation field can be prescribed to excite well defined structures in the subject of the examination.
When utilizing these NMR signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct an image using one of many well known reconstruction techniques. When using these NMR signals in spectroscopy, the magnetic field gradients are employed along with RF saturation pulses to suppress NMR signals from all tissues except the prescribed structure of interest.
It is desirable when imaging a structure embedded in other anatomy to prescribe a pulse sequence that will optimize a scan parameter such as scan time, image resolution, image SNR, or CNR. Similarly, when acquiring spectroscopy data it is desirable to optimize the scan prescription for the particular structure from which the information is sought. To achieve this, the target structure must first be separately identified, or “segmented”, from surrounding tissues. Then, the RF pulses and magnetic field gradients in a chosen pulse sequence must be optimized for the segmented structure.
There are many methods known and used to segment different tissue types or structures in the human body. However, such methods require the acquisition of an image as input to the segmentation process and extensive processing time. Processing times measured in hours are required using present technology. As a result, it is not practical to prescribe imaging or spectroscopy scans that are optimized for a particular structure because the prescan and segmentation steps that precede the scan are too lengthy.